1. Field of the Invention
This invention relates to a light-scanning optical system and also to an image forming apparatus comprising such a light-scanning optical system. More particularly, the present invention relates to a light-scanning optical system that is adapted to realize high definition printing and can effectively avoid any printing slippage in the main scanning direction by partly excluding the incident luminous flux entering the photodetector (BD sensor) for generating write-start position synchronizing signals. Such an optical system may suitably be used for a laser beam printer or digital copying machine.
2. Related Background Art
FIG. 1 of the accompanying drawings is a schematic illustration of a known light-scanning optical system, illustrating a principal area thereof. Referring to FIG. 1, the luminous flux emitted from a semiconductor laser 51 with optical modulation in response to the image information given to it is thinned in terms of its cross section by an aperture stop 52 and transformed into a substantially collimated or converged flux by a collimator lens 53 before entering a cylindrical lens 54. The luminous flux that enters the cylindrical lens 54 is let out without any modification within the main scanning section but focused in the sub scanning section to produce a substantially linear image (running along the main scanning direction) on the deflection surface (reflection surface) 55a of light deflector 55. The elements including the aperture stop 52, the collimator lens 53 and the cylindrical lens 54 are those of the first optical system 62. The luminous flux reflected and deflected by the deflection surface 55a of the light deflector 55 is then focused by an imaging optical system (f.theta. lens) 56 operating as the second optical system onto the surface 57 of a photosensitive drum to produce a luminous spot, which is then made to optically scan the surface 57 of the photosensitive drum in the direction of arrow B (main scanning direction) at a uniform rate as the light deflector 55 is driven to rotate in the direction of arrow A. As a result, an image is recorded on the surface 57 of the photosensitive drum which is a recording medium.
In such a light-scanning optical system, generally, a photodetector is arranged for detecting a write-start synchronizing signal immediately before writing the image signal in order to accurately control the write-start position for writing the image signal.
In FIG. 1, reference numeral 58 denotes a bending mirror (to be referred to as "BD mirror" hereinafter) arranged to reflect the luminous flux for detecting the write-start position synchronizing signal to the BD sensor 61 in order to regulate the timing of spotting the scanning start point on the surface 57 of -the photosensitive drum and reference numeral 59 denotes a slit arranged at a position equivalent to the surface 57 of the photosensitive drum 57. The slit 59 has a width of about 0.5 mm and a luminous flux having a diameter of about 0.1 mm passes therethrough. Reference numeral 60 denotes a BD lens operating as imaging means and arranged to take a role of establishing a conjugate relationship between the BD mirror 58 and the BD sensor 61. It also takes a role of correcting the inclination of the BD mirror 58. Reference numeral 61 denotes a photodetector (to be referred to as "BD sensor" hereinafter) operating as write-start position synchronizing signal detecting means.
Thus, the timing of spotting the scanning start point on the surface 57 of the photosensitive drum is regulated by means of the output signal of the BD sensor 61 in FIG. 1.
Meanwhile, when arranging a light-scanning optical system in the image-forming apparatus main body, the write-start synchronizing signal (to be referred to as "BD signal" hereinafter) may have to be detected at the side opposite to the first optical system 62 relative to the optical axis of the second optical system (f.theta. lens) as shown in FIG. 2 depending on the positional restrictions due to the configuration of the main body and the arrangement of the electrical equipment. Then, the polygon mirror 55 has to be rotated in the direction opposite to that of FIG. 1 and the scanning luminous spot on the plane to be scanned 57 also has to be moved oppositely. Note that, in FIG. 2, the components same as those of FIG. 1 are denoted respectively by the same reference symbols.
In light-scanning optical systems as shown in FIGS. 1 and 2, generally, the margin between the edge of the luminous flux getting to the opposite ends (point U and point L in FIGS. 1 and 2) of the image and the opposite ends in the longitudinal direction (main scanning direction) of the deflection surface 55a of the polygon mirror 55 is disregarded for ensuring good optical performance.
FIGS. 3A and 3B are enlarged views of the deflection surface 55a of the polygon mirror 55, illustrating the margin. FIG. 3A shows the luminous flux reflected by the polygon mirror 55 to get to the point U. The distance between the marginal end of the luminous flux and the corresponding longitudinal end of the deflection surface 55a of the polygon mirror 55 is defined as margin .DELTA.U. Similarly, FIG. 3B shows the luminous flux reflected by the polygon mirror 55 to get to the point L. The distance between the marginal end of the luminous flux and the corresponding longitudinal end of the deflection surface 55a of the polygon mirror 55 is defined as margin .DELTA.L.
In ordinary light-scanning optical systems, the following relationship is normally observed. EQU .DELTA.U&gt;.DELTA.L
Therefore, if the scanning optical system has to be arranged in a manner as shown in FIG. 2, the BD signal has to be detected on the side where the margin of the deflection surface 55a of the polygon mirror 55 is scarce. This means that the scanning angle is limited or the diameter of the luminous flux is limited to minimize the scanning luminous spot to a great disadvantage of the performance of the system.
However, all the luminous flux coming from the first optical system 62 does not necessarily have to be reflected by the polygon mirror 55 so long as the luminous flux getting to the BD sensor 61 has a diameter small enough to pass through the slit 60 and provides a certain level of tolerance to the sensitivity of the BD sensor 61.
Referring to FIG. 4, in known scanning optical systems, it is therefore typically so designed that the polygon mirror 55 is caused to intentionally vignette the luminous flux getting to the BD sensor (not shown) in order to provide a wide scanning luminous spot diameter without vignetting in the effective area of the image, while allowing a wide scanning angle.
However, such known light-scanning optical systems are more often than not accompanied by the problem of printing slippage in the main scanning direction because the quantity of light arriving to the BD sensor fluctuates depending on the deflection surfaces of the polygon mirror due to a possible eccentricity of the axis of rotation of the polygon mirror, uneven accuracy of machining the longitudinal edges of the deflection surfaces of the polygon mirror, the difference in the reflectivity of the films formed by evaporation on the deflection surfaces particularly in areas close to the edges and other factors.
Now, this phenomenon will be discussed by referring to FIGS. 5 and 6. FIG. 5 is a timing chart of a BD signal (BD) and a laser drive signal (LD). Since the polygon mirror is rotating at a constant angular velocity, a BD signal is applied at regular temporal intervals and a laser drive signal is transmitted for a scanning line at predetermined time t.sub.1 after the application of the BD signal for the scanning line. Thus, all the scanning lines are made to have an identical start point. The BD signal is output at time t.sub.0 after the time when the output of the BD sensor gets to a predetermined slice level S as shown in FIG. 6. Thus, the laser drive signal is transmitted at the predetermined time t.sub.1 after this time for a specific scanning line. If the quantity of light getting to the BD sensor fluctuates depending on the deflection surfaces of the polygon mirror for the above described reasons, the time t.sub.0 can vary as a function of the fluctuations of the quality of light getting to the BD sensor to produce a time lag of At as shown in FIG. 6. Then, the transmission of the laser drive signal for the scanning lines also shows a time lag of .DELTA.t to give rise to the phenomenon of printing slippage in the main scanning direction.
A similar problem arises when such a known light-scanning optical system is realized as multi-beam scanning optical system by using a plurality of light sources (light emitting sections).
For instance, when a popular monolithic 2-beam laser (e.g., multi-beam semiconductor laser) is used as light source, the two light emitting spots are separated at least by a distance as large as about 0.1 mm. If the light emitting spots of the light source are arranged perpendicularly relative to the sub scanning direction, the corresponding focused luminous spots are also separated in the sub scanning direction by more than 0.1 mm on the plane to be scanned. If the resolution of the optical system is 600 DPI, the luminous spots have to be separated in the sub scanning direction by 42.3 .mu.m and then the optical system may require the use of a so-called interlace scanning system, which needs a memory for storing data for several lines to be jumped over to consequently raise the overall cost. The use of a costly memory can be avoided by arranging the two light emitting spots A and B of the light source 71 not perpendicularly but with an angle of .theta. relative to the sub scanning direction S that provides a distance between the two luminous spots on the plane to be scanned 57 in that direction that matches the resolution of the optical system as shown in FIG. 7. In FIG. 7, reference symbols 53 and 54 respectively denote a collimator lens and a cylindrical lens while reference symbols 66 and M respectively denote a f.theta. lens and the main scanning direction.
When the light source 71 is arranged in the above described manner, the two luminous fluxes emitted from the two light emitting spots A and B (laser A having the light emitting spot A and laser B having the light emitting spot B) follows the respective optical paths as shown in FIG. 8. If the polygon mirror 55 is caused to intentionally vignette the luminous fluxes getting to the BD sensor as in the case of known light-scanning optical systems, the ratio of vignetting the laser A and that of vignetting the laser B of the polygon mirror 55 are inevitably differentiated to consequently differentiate the output of the BD sensor for the laser A and that of the BD sensor for the laser B. Then, as discussed above, there arises the problem of printing slippage in the main scanning direction. If the difference of the outputs of the two BD sensors is constant, this problem may be dissolved by selecting different values for t.sub.1 for laser A and for laser B, taking the time discrepancy of .DELTA.t into consideration. However, in reality, the difference of the outputs of the two BD sensors is by no means constant and it is highly difficult to completely eliminate the problem of printing slippage in the main scanning direction because the luminous fluxes are displaced longitudinally relative to the deflection surface of the polygon mirror by a minute distance due to an alignment error of the light source and other possible errors. Note that, in FIG. 8, reference symbols 52 and 53 denotes respectively the aperture stop and the collimator lens, while reference symbols 54 and 55a denotes respectively the cylindrical lens and the deflection surface.